Steady-state and pre-steady state kinetic studies on the interaction of actin, myosin-subfragment-one (S-1) and ATP were carries out to better elucidate the mechanism of the actin-activated myosin ATPase. We have already shown that it is not required for myosin to detach from actin during eac cycle of ATP hydrolysis. In the present study, we provide further evidence for the second major feature of our kinetic model -- that there is a special rate-limiting step, whic preceeds Pi release and which occurs with the myosin either attached to or detached from actin. The existance of this special rate-limiting step can be proven by showing that the nucleotide on S-1 exists as bound ADP.Pi rather than ATP during steady-state ATP hydrolysis at high actin concentration. A preparation in which S-1 is covalently bound to actin has been used to demonstrate this point and indeed most of the nucleotide exists as bound ADP.Pi. This strongly suggests that the rate-limiting step in the ATPase cycle occurs after ATP hydrolysis but before Pi release. The importance of this step is that it controls the velocity of muscle contraction.